Although transparent ceramics, quartz glass or other glass material is used for an arc tube of such a short arc lamp in general, a lamp for a special use in which a lamp main body is made from opaque ceramics, and translucent ceramics is used for only the optical extraction section, is known. This lamp has a pillar-shaped appearance as a whole, and is very strong and can be easily dealt, and such a lamp is used as a lamp for medical treatments because it is highly safe.
Such a conventional short arc lamp is explained referring to FIGS. 10A and 10B. FIG. 10A is a cross sectional view of the conventional short arc lamp, and FIG. 10B is a diagram in which only an electric supply ring 203, a support member 210, and a cathode 204 of the short arc lamp shown in FIG. 10A are shown.
The body portion 201 of the short arc lamp 200 is made from an insulating member made of alumina, and a concave electrical discharge space D which is partially defined by a reflective surface 202 is formed therein. In the concave electrical discharge space D, the tungsten cathode 204 and a tungsten anode 205 in which thorium oxide is doped, are arranged at a focal point of the reflective surface 202 with a gap therebetween. A tip edge of the body portion 201, leads out of an opening of a reflective surface 202, in which an electric supply ring 206 made of kovar which has an outer diameter approximately equal to the outer diameter of the body portion 201 is arranged through a ceramic ring 203 thereto. A ring-like flange 207 is inserted in the inside of the electric supply ring 206 so as to be in contact with the inside of the ring 206, and a transparent window member 208 which is made of sapphire is joined to the inner circumference face of the flange 207. A first metal member 209 is provided, so that the electric supply ring 206, the flange 207 which is arranged in contact with the inner circumference face of the ring 206, and the window member 208 are fixed to the body portion 201. In a radius direction of the electric supply ring 206, in the concave electrical discharge space D, molybdenum support members 210 whose current transport property and thermal resistance are considered, extends, one end of which is connected to the electric supply ring 206, and the other end of which is connected to the cathode 204. That is, while the support member 210 forms a path for passing current to the cathode 204, it supports the cathode 204 so as to be arranged at a predetermined position of the concave electrical discharge space D.
Recently, when using such a short arc lamp as a light source of an endoscope, there is a demand that the optical output be increased so as to reproduce an affected part more clearly. As a method of increasing an optical output, the brightness of an arc is raised by shortening a distance between electrodes, or increasing an input current.
In the short arc lamp in which such a measure is taken, there is a tendency that the temperature of an electrode rises, and specifically, the problems set forth below may occur in connection with the temperature rise of the cathode.
As shown in FIG. 10B, the cathode 204 is joined to the end portion side of the support member 210 by brazing which uses wax material. The technology in which the support member is joined with the cathode by the wax material is disclosed in Japanese Laid Open Patent No. H09-161727. Or, although not illustrated, in a state where the cathode 204 and the support member 210 are in contact with each other, a contact portion thereof is heated by laser etc. so that the cathode 204 and the support member 210 are welded and joined to each other. Similarly, the technology in which the support member and the cathode are joined by welding is disclosed in Japanese Laid Open Patent No. H09-161727. In such a structure, the heat of the cathode 204 is transmitted to the support member 210, the heat of the support member 210 is transmitted to the electric supply ring 206, and, finally the heat of cathode 204 is radiated from a first metal member 209.
However, if the temperature of the cathode 204 rises, the heat dissipation effect of the heat dissipation path reaches to the limit, and the heat of the cathode 204 cannot be fully released, so that the temperature of wax material at a coupling area of the cathode 204 and the support member 210 or the temperature of a welding portion rises. This is attributed to the temperature rise of the cathode 204 itself and the temperature rise of the support member 210 due to a heat accumulation therein because the heat which is conducted in the support member 210 is not fully conducted to the electric supply ring 206.
And where stress is repeatedly generated in the wax material or the welding portion, with turning on and off a lamp, so that the temperature of the wax material or the welding portion rises excessively, the cracks are generated in the wax material or the welding portion at an early stage, and if stress is repeatedly generated in the state where these cracks are generated therein, these cracks grow whereby the wax material or the welding portion is destroyed. And when such a destructive state advances, there is a problem that the cathode 204 drops out of the support member 210. Moreover, when the temperature of the wax material becomes 1000 degrees Celsius or more, the wax material melts. When this phenomenon happens, there is a problem that the cathode 204 drops out of the support member 210.
On the other hand, the technology is known in which, not in order to control the temperature rise of the coupling area of the cathode and the support member but in order to control the temperature rise of a joint of the support member and the electric supply ring, the support member is bent or crooked so as to increase the surface area of the support member, thereby positively releasing heat from the supporting member. This technology is disclosed in Japanese Laid Open Patent No. 2005-71684.
However, there are problems that when the support member is bent or curved, it became difficult to position the cathode in a predetermined position, and if the temperature of the support member rises during lighting, the support member is expanded so that the position of the cathode shifts from the predetermined position.